The included zlib should not be used on Linux or OS X, but (depending on
local configuration) it might be needed for crosscompiling a mingw target
from Linux. Now the user can choose whether to use the included zlib or not.
cmake does already support that.
zconf.h.in was taken from the original zlib 1.2.8 package. The generation of
zconf.h was made equivalent to the one generated by cmake.
